block of terrain with topo on top?
Enter edit mode, select the edges all around your mesh:
Use AltRclick to select an edge loop, or CtrlRclick to select a path between your previous selection and the new one.

Once you have selected all that, simply extrude down:
E for extrude, press Z to constrain it to the Z axis, then validate with Lclick.

Then make this extrusion flat by scaling it on Z to zero:
S for scale, then Z, then Numpad 0 and validate.

Not you just have to make a face from that edge loop, with F.





EDIT :



It looks like your mesh is deformed by a modifier, I would guess a displace modifier. I guess it's how you obtained the shape of the top. The issue is that the modifier affects your whole object, even the parts you want to keep flat.

So we need to restrain the area of action of this modifier.
A simple way to do that is to use a vertex group.

For the sake of demonstration, I recreated your setup (at least what I guessed out of your pictures):



As you can see on the right, I show both the modifier settings and the "vertex group" panel which can be found in the Object Data tab (the triangle icon).

First, let's create the vertex group we need:


Enter edit mode
In vertex selection, select only the top part of the mesh we want to deform
Hit CtrlG to assign the vertices to a new group, hit the confirmation button spawned under your cursor.
You can see the created group in the vertex group panel, I rename it "Deform Mask" for the sake of clarity.
In the modifier's settings, select the Deform Mask group in the vertex group input.


Now it deforms only the top vertices, but we need to do one more thing. You will notice that the top vertices are still being moved on X and Y axes, making the vertical faces deformed instead of flat.
This is because of the direction of the deformation. By default, this modifier deforms in all directions according to the faces's normals. We want in this case to move vertices only up and down, so let's change the direction to Z.

And here we go: